Sulfur-containing crude oil is referred to as “sour” and numerous processes have been described for “sweetening” the crude oil to reduce its sulfur content. Traditional hydrotreating is suitable for oil fractions, but not for whole crude oil. Treatment by separation alone leads to a loss of the crude oil volume.
There are practical methods for the desulfurization of fractions of crude oil. Various approaches have been suggested in the prior art for the desulfurization of crude oil, but there are technical difficulties and the associated costs are high. Processes for very heavy crude oils include the combination of desulfuring and cracking to produce synthetic crude.
By way of background, U.S. Pat. No. 6,955,753 discloses a process by which sulfur compounds and metals are extracted to aqueous-based solvents after a chemical reaction with an acid or a base. An emulsifier is also required to increase the contact surface area between the aqueous solvent and the oil.
In U.S. Pat. No. 5,582,714, the extraction of sulfur compounds from previously hydro-treated fractions is described. The fractions must be more volatile than the solvent in this process so that in the solvent regeneration step the sulfur compounds are vaporized, and the solvent remains a liquid. The relatively small volume of the sulfur-containing solvent stream of this process is due to the small amount of sulfur compounds in gasoline compared to the sulfur content of crude oil or heavy oil fractions. Table 1 of the patent shows that the gasoline treated 0.0464% sulfur compared to the average of 3% sulfur present in Arabian heavy crude oil.
The solvent extraction process disclosed in U.S. Pat. No. 4,385,984 is directed to reducing the polyaromatic compounds and increasing the oxidation stability of lubricating oils. Solvent recovery is not described.
A double solvent extraction process is disclosed in U.S. Pat. No. 4,124,489 for the purpose of reducing the polyaromatics content and increasing the oxidation stability of the oils. Sulfur reduction is a byproduct of the polyaromatics removal.
These processes are not suitable for, or readily adapted to the treatment of whole crude oil and other heavy fractions having a relatively high naturally-occurring sulfur content.
It is therefore one object of the present invention to provide an improved continuous process for extractive desulfurization of crude oil in which all or a substantial proportion of the solvent is recovered and recycled for use in the process.
Another object of the invention is to provide an improved continuous solvent extraction process that can be used to substantially reduce the sulfur content of crude oil and other untreated hydrocarbon streams that have a high natural sulfur content.
A further object of the invention is to provide a process for reducing the sulfur content of a crude oil feed stream that minimizes the capital requirement by utilizing existing equipment and well established procedures in one of the process steps.
Yet another object of the invention is to provide an improved solvent extraction process in which the solvent or solvents employed can be vigorously mixed with a crude oil, or a crude oil fraction, without forming an emulsion and that will provide clear liquid-liquid phase separation upon standing.